This invention is related to the field of drill bits used to drill wellbores in the earth. More specifically, the invention is related to structures for roller cone drill bits which have improved drilling performance.
Roller cone rock bits and fixed cutter bits are commonly used in the oil and gas industry, as well as in the mining industry, for drilling wellbores through earth formations. FIG. 1 shows one example of a conventional drilling system used to drill such a wellbore. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into the well bore 14. Connected to the end of the drill string 12 is a roller cone-type drill bit 20, shown in further detail in FIG. 2.
The roller cone bit 20 typically includes a bit body 22 having an externally threaded connection at one end 24 for coupling to the drill string (12 in FIG. 1), and a plurality of roller cones 26 (usually three as shown) attached to the other end of the bit 20 and able to rotate with respect to the bit body 22. Attached to the cones 26 of the bit 20 are a plurality of cutting elements 28 typically arranged in rows about the surface of each of the cones 26. The cutting elements 28 can be tungsten carbide inserts, polycrystalline diamond compacts, or milled steel teeth.
As is known in the art, the drilling system typically includes apparatus for circulating drilling fluid through the drill string (12 in FIG. 1) and the bit 20 to cool the bit and to lift cuttings out of the wellbore (14 in FIG. 1). For wellbores drilled to extract oil and gas, the drilling fluid is typically xe2x80x9cmudxe2x80x9d or similar liquid. For mining applications, the drilling fluid is often compressed air. The principles of roller cone bit design are similar in either case.
Drill bits are classified and selected for use according to the characteristics of the earth formations that are expected to be drilled with the particular drill bit. A drill bit classification system has been adopted by the International Association of Drilling Contractors (IADC) which includes a 3-digit identification number to characterize drill bits according to the formations expected to be drilled. Formations having increasing hardness are generally drilled by bits having higher numbers in the classification. The first number in the IADC code is called the xe2x80x9cseriesxe2x80x9d and is related to the type of cutting element on the roller cones. First numbers in the range 1-3 are xe2x80x9cmilled toothxe2x80x9d bits, while first numbers in the range 4-8 are xe2x80x9cinsertxe2x80x9d type bits. The first number (the series) increases as the hardness of the formation to be drilled increases. The second number in the classification is related to the bit type within the series. Because the third number of the IADC code relates only to bearing design and gage protection, it is omitted herein, as extraneous. Harder formations are typically drilled with bits having a higher second number classification. For example, a drill bit in IADC class 5-3 is used to drill harder formations than a bit in IADC class 5-2.
Generally, roller cone drill bits known in the art having IADC classification of 6-1 and higher have particular structural characteristics (design parameters) believed to be advantageous when drilling the formations for which these bits are intended. One such design parameter is the xe2x80x9cjournal anglexe2x80x9d, which is defined as an angle subtended between the axis of rotation of the roller cones and a plane perpendicular to the axis of rotation of the drill bit. Prior art bits of IADC class 6-1 and higher typically have a journal angle of about 36 degrees or more. Softer formation bits (typically in IADC classes lower than 6-1) have journal angles of about 32 to 33 degrees.
Another design parameter of roller cone drill bits is called xe2x80x9coffsetxe2x80x9d, which is defined as the separation between the rotational axis of each roller cone and a line perpendicular to the axis of rotation of the bit which intersects the axis of rotation of the bit (meaning a line extending radially outward from the axis of rotation of the bit). Typical prior art drill bits used to drill harder formations (IADC class 6-1 and higher) have offset of about 0.125 inches (3.2 mm). Softer formation bits have offset of at least about 0.219 inches (5.6 mm).
Another design parameter is known as xe2x80x9coversize anglexe2x80x9d, which is defined as the angle subtended between a line perpendicular to the axis of rotation of the bit, and a line connecting two specific points. The first specific point is the intersection of the rotation axis of one of the roller cones and a plane perpendicular to the axis of rotation of the bit. The second specific point is the point of contact between the cutting elements in an outermost row of cutting elements, called the xe2x80x9cgage rowxe2x80x9d, and a curve known as the xe2x80x9cgage curvexe2x80x9d. Calculation of the gage curve is known in the art, and is described, or example in U.S. Pat. No. 5,833,020 issued to Portwood et al. Typical prior art hard formation bits (IADC class 6-1 and higher) have oversize angles in a range of about 1 to 1.5 degrees. Soft formation bits have oversize angles typically greater than about 2 degrees.
Prior art roller cone drill bits are generally designed by testing a selected design under actual drilling conditions. The drilling performance and wear characteristics of the selected bit design are compared with those of bits having other designs. Because of the large number of design parameters in the typical roller cone drill bit, it has been impractical, using prior art design techniques, to test all of the design parameters on a drill bit. As a result, typical prior art roller cone bits have journal angles, offset and oversize angles which are carried forward from previous bit designs. Journal angles, offset and oversize angles of prior art bits may not always provide optimal drilling performance. It is desirable to have a drill bit in which journal angle, offset and/or oversize angle have been determined to provide better drilling performance.
One aspect of the invention is a roller cone drill bit which includes at least one roller cone rotatably mounted on a journal forming a part of a bit body. The at least one cone has cutting elements disposed at selected locations thereon. The at least one roller cone has a journal angle of less than about 35 degrees, and an offset less than about 0.15 inches.
In one embodiment, gage row cutting elements on the at least one roller cone define an oversize angle in a range of about xe2x88x921.5 to +2 degrees.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.